1. Field of the Invention
The present invention relates to an apparatus and method for kneading rubber or a rubber composition, for the purpose of masticating rubber to improve fluidity and workability, and the purpose of kneading a material to be kneaded, which contains rubber and various compounding agents.
2. Description of the Related Art
In producing a kneaded product of a rubber composition comprising rubber and various compounding agents, particularly in producing a tire rubber compound mainly comprising natural rubber, a batch system is frequently used, in which predetermined amounts of raw materials are intermittently kneaded one time each. In this case, batch-system kneading is repeated until extrusion workability and a degree of dispersion of a compounding agent reach desired levels, i.e., desired levels of viscosity and of dispersion are obtained. This kneading step is generally referred to as a “re-milling step”, and uses a batch-system kneading apparatus such as an internal mixer or the like.
However, in kneading by using such a batch-system kneading apparatus, the kneaded product must be transferred to an extruder from the kneading apparatus by using a transport device after the kneading step, and must be molded in a desired shape, thereby causing the need for a step of transferring a bulk kneaded material. Therefore, the apparatus is easily complicated and increased in size, and a manual work by a worker is required, failing to achieve power saving. Also, in kneading by the batch-system kneading apparatus, it is necessary to prevent an excessive increase in temperature during kneading in order to prevent deterioration and improve a kneading efficiency. Therefore, the bulk kneaded material after kneading is formed in a sheet and cooled, and again subjected to a re-milling step several times. The re-milling step is repeated until the viscosity is decreased to a desired level, and in some cases, the re-milling step is performed about 5 times. This is undesirable from the viewpoint of improvement in productivity.
On the other hand, from the viewpoint of improvement in productivity, a method of continuously kneading a rubber composition is disclosed (Japanese Unexamined Patent Application Publication No. 11-262945). This uses, for kneading rubber, a double-screw kneading extruder, which is a typical kneading extruder for plastic compositions. The double-screw kneading extruder is provided with a rubber charging port to which a rubber supply extruder is further connected so that a rubber composition is continuously supplied and continuously kneaded by the double-screw kneading extruder.
However, in the continuous kneading apparatus disclosed in the above publication, a kneading state, i.e., a viscosity level and a dispersion level of a compounding agent, can be controlled in only a limited range which permits control by a means for controlling the rotational speed of the double-screw kneading extruder, or the like. The viscosity level and the dispersion level cannot be easily controlled in such a wide range as controlled by a batch-system kneading apparatus. The shape of a screw can be changed so that the kneading apparatus can be widely applied to rubber compositions having various viscosities. However, the screw of the double-screw kneading extruder comprises a segmented kneading blade (a rotor segment, kneading disk, a screw segment, or the like), which is combined with a splined shaft, and thus in a large-size equipment, much labor and time are required for changing the shape or configuration the screw.